If a lens of refractive index µ is immersed in a medium of refractive index µ1, then its focal length in medium is given by
1
fm
=(mµl−1)(
1
R1
−
1
R2
) If fa is the focal length of lens in air, then
1
fa
=(aµl−1)(
1
R1
−
1
R2
) ⇒
fm
fa
=
(aµl−1)
(mµl−1)
If µ1>µ, then fm and fa have opposite signs and the nature of lens changes i.e. a convex lens diverges the light rays and concave lens converges the light rays. Thus given option (a) is correct.